The present invention generally relates to electronic test devices for testing electronic circuits to be tested, and relates in particular to a method and a circuit for protecting sensitive data during scan mode of the test device. Electronic circuits that have to be tested may contain sensitive information which should not be retrieved during a scan mode of such a testing procedure. Unauthorized use of such a scan test operation of the integrated circuit may result in loss or unauthorized use of sensitive information.
In order to protect this kind of sensitive information, it has been proposed to introduce a test isolation multiplexer into a protected register's chain. FIG. 1 shows a protected register's chain wherein an output signal of the protected register's chain can only be read out if the scan mode is not entered. If the scan mode is entered, i.e. scan mode=1, then the test isolation multiplexer M blocks the output signal O from the protected register's chain.
A major disadvantage of the configuration of the state of the art shown in FIG. 1 is that registers are excluded from the scan mode thus that their functionality cannot be checked. Thus, the registers excluded by the test isolation multiplexer are disadvantageously not tested.